1. Field of the Invention
The present invention relates to a sheet post-processing apparatus for squaring a spine of a sheet bundle and an image forming apparatus including the same. More particularly, it relates to a sheet post-processing apparatus for dressing up a booklet by performing squaring of a spine of a saddle stitch bound booklet.
2. Description of the Related Art
Conventionally, when sets of approximately more than 20 sheets are folded in one lump sum, booklet is obtained that apparently has a curve adjacent to the spine. When such booklet is folded, an end portion on an opposite side of the spine will instantly open so that such booklet has poor-looking appearance. Moreover, since such booklet cannot be kept in flat conditions, it is hard to stack a plurality of booklets.
To address this issue, an apparatus and a method for pressing a curved spine of booklet and angularly deforming the spine 2a1 for squaring is suggested in U.S. Pat. No. 6,692,208. According to the technique of U.S. Pat. No. 6,692,208, as illustrated in FIG. 24A, a spine 2a1 of a booklet 2a is set to the leading position to be conveyed by conveying units 706, 707, and the spine 2a1 of the booklet 2a is struck by a positioning unit 705 for positioning the same. The booklet 2a is then pinched by holding units 702, 703 for fixing, the positioning unit 705 is retracted, and a pressing roller 704 allows the booklet to run along the spine 2a1 while applying pressure to the spine 2a1 of the booklet 2a. In this manner, the curved spine 2a1 undergoes squaring. FIG. 24B is a pattern diagram illustrating a running and moving direction A of the pressing roller 704, wherein the pressing roller 704 is retracted to an area in which it does not contact the booklet 2a until the booklet 2a is pinched by the holding units 702, 703. When the booklet 2a is pinched and fixed, the roller moves from one end to the other end of the booklet 2a while applying pressure to the spine 2a1.
However, according to the technique of U.S. Pat. No. 6,692,208, the pressing roller 704, including rotating members, rotates and presses the spine 2a1 of the booklet 2a that is held by the holding units 702, 703 while it moves along the spine 2a1 such that the back portion of the booklet 2a is deformed and squared. While the booklet 2a is held by the holding units 702, 703 when the spine 2a1 of the booklet 2a is pressed by the pressing roller 704, the pressing surface of the booklet 2a is pushed downstream of the running and moving direction A of the pressing roller 704. Therefore, as illustrated in FIG. 24C, wrinkles C are generated on the booklet 2a in oblique directions between points that are held by the holding units 702, 703 and the pressing surface that has been pushed downstream by the pressing roller 704 such that the appearance is worsened.
At this time, directions of the wrinkles C vary depending on the moving direction of the pressing roller 704, and the larger the pressing amount of the pressing roller 704 at the spine 2a1 of the booklet 2a is, the larger the pressing force of the pressing roller 704 becomes. Here, the pressing amount of the pressing roller 704 is distance from spine 2a1 prior to squaring up to a pressing surface of the pressing roller 704 when squaring processes is performed the spine 2a1. Accordingly, the pressing surface of the booklet 2a is easily pushed downstream of the running and moving direction A of the pressing roller 704 so that generation of wrinkles becomes strongly apparent. For restricting opening of the end portion opposite of the spine 2a1 so as to keep the booklets 2a in flat conditions and to stack a plurality of booklet 2a necessitates reliable squaring so that it is necessary to set a large pressing amount for the pressing roller 704 at the spine 2a1 of the booklet 2a. Accordingly, when the roller is pressed at the predetermined pressing amount, wrinkles C are generated so as to worsen the appearance of the booklet 2a. 
The present invention accordingly aims to solve the subject of squaring spine in a good-looking manner without leaving “wrinkles” on back portions of booklet.